A polar bear is shown in the Arctic National Wildlife Refuge in Alaska, where summer sea ice has dramatically decreased in recent years. / SUBHANKAR BANERJEE, AP

by Wendy Koch, USA TODAY

by Wendy Koch, USA TODAY

Climate change is often called a "defining" issue of our time but given its controversy, it can be difficult to understand. So on Thursday, two of the world's leading scientific groups released a reader-friendly guide.

This primer explains why, despite a cold winter in parts of the United States, scientists are so sure that man-made global warming is worsening, and how that's melting Arctic sea ice, raising sea levels and acidifying the oceans.

"There will always be cold nights and cold days...but they'll become rarer," co-author Inez Fung of the University of California-Berkeley said at a Washington, D.C., forum to unveil the publication. She also said a warmer, moister atmosphere provides more energy for storms and severe weather events.

"Actions today have long-term consequences," Fung said, noting how heat-trapping carbon emissions from burning fossil fuels linger a long time in the atmosphere. Even if those emissions suddenly stop, she said it will take thousands of years for the Earth to cool to pre-industrial levels.

Co-author Eric Wolff of the University of Cambridge said the planet's climate has changed throughout history but now it's changing 10 times faster, making adaptation more challenging. He said most of the warming (1.4 degrees Fahrenheit) since 1900 has occurred since the mid-1970s.

Climate Change: Evidence and Causes is written by a dozen climate scientists on behalf of the U.S. National Academy of Sciences, a private independent group, and the Royal Society, the national scientific academy of the United Kingdom. It explains what scientists know and what they're still learning, such as the intricacies of clouds.

The authors say carbon dioxide (CO2) concentrations are now higher than at any time in at least 800,000 years, and unless carbon emissions are curbed, the planet could warm 4.7 to 8.6 degrees Fahrenheit by 2100. They answer 20 key questions, 10 of which are excerpted below:

How do scientists know that recent climate change is largely caused by human activities?

Direct measurements of CO2 in the atmosphere and in air trapped in ice show that atmospheric CO2 increased by about 40% from 1800 to 2012. Measurements of different forms of carbon reveal that this increase is because of human activities.

CO2 is already in the atmosphere naturally, so why are emissions from human activity significant?

Human activities have significantly disturbed the natural carbon cycle by extracting long-buried fossil fuels and burning them for energy, thus releasing CO2 into the atmosphere.â??

What role has the sun played in climate change in recent decades?

The sun provides the primary source of energy driving Earth's climate system, but its variations have played very little role in the climate changes observed in recent decades. Direct satellite measurements since the late 1970s show no net increase in the sun's output, while at the same time global surface temperatures have increased.

Is there a point at which adding more CO2 will not cause further warming?

No. Adding more CO2 to the atmosphere will cause surface temperatures to continue to increase. As the atmospheric concentrations of CO2 increase, the addition of extra CO2 becomes progressively less effective at trapping Earth's energy, but the surface temperature will still rise.

Does the recent slowdown of warming mean that climate change is no longer happening?

No. Since the very warm year 1998 that followed the strong 1997-98 El NiÃ±o, the increase in average surface temperature has slowed relative to the previous decade of rapid temperature increases. Despite the slower rate of warming, the 2000s were warmer than the 1990s. A short-term slowdown in the warming of Earth's surface does not invalidate our understanding of long-term changes in global temperature.

If the world is warming, why are some winters and summers still very cold?

Global warming is a long-term trend, but that does not mean that every year will be warmer than the previous one. Day-to-day and year-to-year changes in weather patterns will continue to produce some unusually cold days and nights, and winters and summers, even as the climate warms.

Why is Arctic sea ice decreasing while Antarctic sea ice is not?

Sea ice extent is affected by winds and ocean currents as well as temperature. Sea ice in the partly-enclosed Arctic Ocean seems to be responding directly to warming, while changes in winds and in the ocean seem to be dominating the patterns of climate and sea ice change in the ocean around Antarctica.

How fast is sea level rising?

Long-term measurements of tide gauges and recent satellite data show that the global sea level is rising, with best estimates of the global average rise over the past two decades centered on 3.2 mm per year (0.12 inches per year). The overall observed rise since 1901 is about 20 cm (8 inches).

What is ocean acidification and why does it matter?

Direct observations of ocean chemistry have shown that the chemical balance of seawater has shifted to a more acidic state. Some marine organisms (such as corals and some shellfish) have shells composed of calcium carbonate, which dissolve more readily in acid. As the acidity of seawater increases, it becomes more difficult for them to form or maintain their shells.

Are climate changes of a few degrees a cause for concern?

Yes. Even though an increase of a few degrees in global average temperature does not sound like much, global average temperature during the last ice age was only about 4 to 5 Â°C (7 to 9 Â°F) colder than now. Global warming of just a few degrees will be associated with widespread changes in regional and local temperature and precipitation as well as with increases in some types of extreme weather events.